Migraine and tension-type headache (TTH) can increase in frequency and transform from episodic to chronic forms. The process of transformation of these primary headaches is complex and involves multiple risk factors. In this cross-sectional and longitudinal population study, we aimed to investigate the relation of clinical characteristics of primary headaches to poor outcome: new-onset or persistent chronic headache (> or =180 days/year). Individuals who had migraine + / - TTH and those who had pure TTH were studied separately. Of 740 individuals who entered this study in 1989, 673 were eligible for follow-up in 2001, and a total of 549 individuals participated in the follow-up study. At baseline in 1989, no difference was found between episodic and chronic migraine headache ( + / - TTH). Duration of headache episodes >72 hours (p = .002) was associated with pure chronic TTH at baseline in 1989 whereas aggravation of headache by physical activity (p = .045) was associated with pure frequent episodic TTH. Of 64 subjects with migraine + / - TTH, 12 had a poor outcome in 2001. For pure TTH, of 116 subjects at baseline, 11 had a poor outcome in 2001. Using multivariate logistic regression analysis with adjustment for medication overuse and use of preventive medications, poor outcome of migraine + / - TTH tended to be associated with a baseline pulsating quality and severe intensity of migraine, photophobia and phonophobia, as well as longer duration of an individual headache attack. For pure TTH, unilateral headache, nausea and individual headache attack duration greater than 72 hours was associated with poor outcome. Pooled data univariate analysis revealed that nausea, daily use of acute headache medications, use of headache preventive medications and coexistent headaches were significant predictors of chronic headache in 2001(p